Typically, commercial aircraft normally have a constant frequency generating system where the frequency of the voltage and current being sent through the feeder to the distribution system, and ultimately to the load, is kept constant within a narrow range, around 400 Hz. More recently, however, variable frequency systems are being incorporated into aircraft. These systems allow the frequency to range from as low as 375 Hz to 810 Hz while still maintaining the voltage magnitude at a constant level.
The variable frequency generator has a synchronous generator that generates an output voltage that is regulated by an exciter field current. The output voltage of the synchronous generator at the point of regulation (POR) is regulated by a generator control unit (GCU) that controls the exciter field current. The exciter field current in variable frequency generating systems is typically controlled by two solid state switches, such as MOSFET switches, that are simultaneously turned on and off via a pulse width modulated (PWM) signal that is sent to the gates of the switches. The duty cycle of the PWM signal is typically determined by a current loop circuit.
Because the variable frequency generator can generate high voltages, particularly when operating in the high frequency range, excessive voltage can build up to undesirably high levels at the POR due to single point failures, such as loss of three-phase voltage sensing capabilities, switch failure, or exciter current loop failure, within the generator control unit.
There is a desire for a system that can detect failures in a variable frequency generating system and prevent excessive voltage buildup at the POR caused by the failures.